1. Field of the Invention
This invention relates to a light-transmissive recording medium to be used suitably for ink jet recording, particularly to a light-transmissive recording medium satisfying both water resistance of the recorded image and ink absorption characteristic.
2. Related Background Art
Ink jet recording performs recording by generating small droplets of ink according to various ink (recording liquid) discharging systems, such as the electrostatic attraction system, the system of giving mechanical vibration or displacement to ink by use of a piezoelectric element, the system of utilizing the pressure of bubbles formed by heating, permitting the droplets to fly and to attach a part or whole thereof onto a recording medium such as paper. This method is now attracting attention as a recording method which generates less noise and is capable of performing high speed printing and multicolor printing.
As the ink for ink jet recording, there have been used those composed mainly of water primarily for the reasons of safety and recording characteristics, and in most cases, a polyhydric alcohol is added for prevention of clogging of nozzles and improvement of discharging stability.
As the recording medium to be used for the ink jet recording method, there have been employed recording media comprising a porous ink receiving layer provided on a conventional paper or a substrate known as the ink recording paper.
However, as the ink jet recording is improved in performance such as speed-up or multi-colorization of recording and becomes wide-spread, higher and broader characteristics are becoming demanded also for the recording medium.
More specifically, the recording medium for ink jet recording for obtaining recorded images of high resolution and high quality is required to satisfy various basic requirements as follows:
(1) reception of ink onto the recording medium should be as rapid as possible;
(2) the ink attached later should not penetrate into the dot attached earlier, even when the ink dots overlap each other;
(3) ink droplets should not diffuse on the recording medium and should not give a larger ink dot diameter than is necessary;
(4) the shape of an ink dot should be approximate to a true circle and its circumference should be smooth;
(5) OD (optical density) of an ink dot should be high, without obscurity in the periphery of the dot; etc.
Further, for obtaining a recorded image quality of high resolution comparable to color photography by the multi-color ink jet recording method, the following performances are required in addition to the above requisite performances:
(6) the colorants of the ink should be excellent in color forming property;
(7) ink fixing characteristic should be excellent since the same number of liquid droplets as there are ink colors may be attached on one spot;
(8) the surface should have lustre;
(9) the degree of whiteness should be high.
Further, while the recorded images obtained by the ink jet recording method have been employed in the past exclusively for surface image observation, recording media suited for uses other than for surface image observation are becoming demanded with improvement in performances or wide usage of the ink jet recording device.
Uses of the recording medium other than for surface image observation may include those in which recorded images are projected onto a screen, etc. by means of an optical instrument such as a slide or an OHP (an overhead projector), etc. and those images are observed, color resolution plates during preparation of a positive plate for color printing, and a CMF (a color mosaic filter), etc. to be used for color display such as liquid crystal, etc.
In surface image observation, the diffused light of the recorded image is observed, while the transmitted light through the recorded images becomes a problem in the recording medium in other uses. Accordingly, light transmissivity, particularly excellent linear transmittance is required in addition to the requisite performances in general of the recording medium for ink jet recording.
However, until the present invention, no recording medium satisfying all of these requisite performances has yet been known.
In the prior art, it has been known to use a water-soluble polymer for formation of an ink receiving layer.
Such a recording medium, while it is excellent in ink absorption, becomes sticky on the surface of the ink receiving layer under highly humid conditions, whereby there are involved the problems such that it adheres to the delivery roller of the printer when mounted on a printer, thus failing to be conveyed, and also that the recording medium will be subject to blocking when placed one upon another.
It has been also known to make the ink receiving layer water-insoluble by addition of a water-resistance-improving agent into an ink receiving layer containing a water-soluble polymer and a pigment.
However, according to such a method, the water-soluble polymer is used merely as the binder for a pigment, etc., and the water-soluble polymer itself does not absorb and hold the ink.
Thus, when such a method is employed, the water-soluble polymer itself is made water-insoluble to give no stickiness; but, on the other hand, ink absorbability of the water-soluble polymer is also lost.
The water-absorbable polymer used for sanitary articles or paper diapers is also obtained by crosslinking a water-soluble polymer to make it water-insoluble. Such crosslinked polymer has no film forming property, and the water absorption characteristic is markedly lowered by the polyhydric alcohol, etc. contained in the ink. Thus, it was unsuitable for a recording method using of an ink.